It has been known for many years that detailed knowledge of the magnetic field of the earth at particular locations is of interest and of value in determining the geological structure of the earth. While the global magnetic field varies generally from the north magnetic pole to the south magnetic pole, local variations in the global magnetic field can be used as clues to deduce the underlying geological structure of the earth and this can be used in exploration for oil, gas and other minerals.
It is well understood that oil and gas are typically found in beds of sedimentary rock. Sedimentary rock is generally rock which has been washed down from mountain peaks and the like and deposited into beds over many millenia of time. Typically, the sedimentary rock does not have a strong magnetic field, while igneous rock, such as the rock from which the sediments are washed, possesses a stronger magnetic field. Accordingly, if one measures the earth's magnetic field above a bed of sedimentary rock deposited over a layer of igneous rock, such as the ocean bed, the intensity of the magnetic field is generally the sum of the earth's global or "ambient" field plus a term proportional to the distance of the underlying igneous rock from the magnetometer used to record the magnetic field. If one measures the field at spaced locations along a line of exploration and finds that the magnetic field intensity varies along that line, one may assume that the points at which the magnetic field is higher correspond to points where the magnetic "basement" of igneous rock more closely approaches the magnetometer. If spaced lines over an area of interest are thus explored, a map can be constructed yielding an approximate picture of the contours of the "basement". This provides a corresponding topological map of the lower contour of the sedimentary rock, which can be compared to an ordinary map to yield a cross-sectional view of the thickness of the sediments along the lines of exploration and hence indicating, among other things, where more or less sedimentary rock is present, which maps can then be interpreted by geologists in the search for oil and gas.
Accordingly, it has become increasingly common that magnetic surveys are performed. The usual practice is simply to fly an airplane towing a magnetometer behind it along spaced parallel lines of exploration, typically a kilometer apart, and record the intensity of the earth's magnetic field at locations spaced, again typically a kilometer apart, along the lines. Cross lines are also flown, for example, spaced six or seven kilometers from one another, to insure that no overall variation in the field goes undetected. The values of the magnetic field thus generated can be plotted on paper; if contour lines connecting points of equal magnetic field are drawn, an overall picture of the structure of the magnetic basement of the earth's surface at that point is the result.
The prior art shows numerous processing methods for increasing the accuracy with which the displays just described correspond to the actual magnetic structure of the earth. One important method of such processing is referred to as reduction-to-the-pole processing. This method refers to the well known fact that the accuracy of the results of plotting the measurements of the magnetic field with respect to the actual geologic structure is influenced by the magnetic latitude at which the measurements are taken. That is to say, the magnetic field recorded with respect to a particular geologic anomaly, i.e., local variation from the mean amount of magnetic material present in the earth's crust at the area of exploration, is influenced by the latitude with respect to the magnetic equator at which the survey is flown. If the anomaly is located at or near the north or south magnetic pole, and corresponds to an increase in magnetic material in the area of exploration, the magnetic field will show its peak value with respect to the ambient field when the magnetometer is directly over the geologic body producing the magnetic anomaly. As the exploration moves toward the magnetic equator, the simple peak shape of the anomaly when recorded with respect to a magnetic body at the pole becomes instead a positive peak and a negative trough in the magnetic field recorded. The relative intensities of the peak and trough vary with latitude. Furthermore, the zero point of the paired peaks does not in general coincide with the epicenter of the magnetic material in the geologic body. Accordingly, the displays of magnetic data taken at points other than at the poles are distorted and do not directly represent the geologic structure of the earth. Finally, at the equator, an increase in magnetic material becomes a trough centered over the epicenter of the magnetic material in the body.
The prior art shows methods by which such displays can be processed mathematically so that the displays generated thereby are comparable to displays which would be generated if the anomaly were located instead at the pole, and hence so that the region of greatest depature of the local magnetic field from the earth's ambient field appears to be located directly over the magnetic body producing the anomaly in the magnetic field. One such method, known as reduction-to-the-pole, is well understood in the prior art, and has been extensively practiced thereby. However, it has been generally recognized that the reduction-to-the-pole method which is commonly used is only useful for data recorded at magnetic latitudes greater than about 25.degree. from the magnetic equator. At latitudes less than about 25.degree., the filtering method generally employed becomes mathematically unstable and the data is undesirably distorted leading to poor correspondence with the underlying geologic structure. The art has recognized the need for an improvement on the reduction-to-the-pole method, particularly in the region of the magnetic equator, and various methods have been tried. (See, for example, Pearson et al, Reduction-to-the-Pole of Low Latitude Magnetic Anomalies, a paper which was presented at the Fall, 1982, meeting of the Society of Exploration Geophysicists in Dallas, Tex. While not prior art against the present application, this paper is instructive in that it expands upon the difficulty of application of reduction-to-the-pole techniques near the magnetic equator, and exemplifies the need of the art for improved methods of processing this magnetic data.) However, none of these methods have been particularly successful and they have generally involved compromises in the signal-to-noise ratio of the recorded data, increased computation time, or the like.